Offshore pipelines must be shut down regularly for maintenance purposes. A maintenance period offshore involves a complete shut-down for typically 14 days. During this period the pipe between offshore and onshore installations will be shut from both sides. Existing pressure sensors at both installations are positioned outside the valves, and thus there are currently no way to monitor the pressure inside the closed pipe to detect a leakage during the shutdown period. It is also important to monitor and control gas pressure during the pipe shutdown process due to the fact that high pressure differences cause wear of the valves, and should be minimized.
From U.S. Pat. No. 3,977,252 there is known an apparatus for monitoring the pressure of a liquid inside a pipeline. The pressure is monitored by passing regularly repeated pulses of ultrasonic waves from a transmitter through the pipe wall, liquid and opposite side of the pipe wall to a receiver. The pressure is determined from the travel time of the pulses. Such an apparatus may find its application in monitoring pressure inside liquid-filled pipes. However, in a gas-filled pipe, the large acoustical impedance contrast between the pipe wall and the gas inside means that only a small portion of the acoustical energy is transferred through the gas. Most of the energy is transferred in the pipe wall directly from the transmitter to the receiver, the wall acting as a resonant waveguide for the acoustic energy. Pulses going the long path along the pipe wall will arrive earlier than pulses passing through the gas due to the large acoustic velocity in steel, and with much larger amplitude, it will completely mask the desired pulses that have passed through the gas. Furthermore would it be impossible to use this method in a multiple phase fluid or a mixture of fluids, as the velocity would differ according to which fluid or phase the waves pass through.
From WO 2014/004620 there is known a method for measuring the pressure inside a vessel using ultrasonic means. An ultrasonic signal is transmitted from a transmitter mounted on the wall of the vessel and received by a receiver mounted on the wall in a distance from the transmitter. A control- or processing unit is used to find the travel time of the signal from the transmitter to the receiver, and the pressure is computed as a function of the travel time.
Thus, there is a need for a method for measuring the pressure inside a pipeline or other container without contacting the interior space of said pipeline or container, and that can be used to measure the pressure reliably even if there should be a gas inside the pipeline or container, that is to provide reliable pressure data independently of which fluid or fluids are present inside the pipe or container. The method should provide a more accurate pressure reading than prior art methods.
It is an object of the present invention to provide an apparatus and method solving said needs. This is achieved in an apparatus and method as claimed in the appended claims.
In particular the invention relates to an apparatus for measuring the internal pressure of a pipe or container, the apparatus including in an acoustical transmitter mounted on a wall of said pipe or container, a signal generator connected to said transmitter and which is adapted to provide a signal to the transmitter, a first receiver mounted on said pipe or container in a distance from the transmitter, a second receiver mounted on the pipe or container in a further distance from the transmitter, and a processing unit connected to said first and second receiver, the processing unit being adapted to measure the travel time of an acoustical signal propagating from the first to the second receiver in the wall and determine the pressure inside the pipe or container from said travel time.
The use of two receivers means that the travel time may be measured with increased accuracy.
The processing unit is adapted to determine the pressure inside the pipe or container from the relation
  P  ∝            2      ⁢      Δ      ⁢                          ⁢      Lt              Δτ      ⁢                          ⁢      D      
where P is the pressure, ΔL is the distance between the first receiver and the second receiver, t is the wall thickness, ΔT is the travel time, and D is the internal diameter of the pipe or container.
According to a preferred embodiment, the signal generator is adapted to emit a signal exciting a thickness mode of said wall.
This will greatly increase the coupling between the transducers.
The processing unit may also be adapted to determine the travel time by cross correlating the received signals.
The invention is intended to be used in noisy environments, and the use of cross-correlation will increase the accuracy of the determination of the travel time between the receivers.
The invention also relates to a corresponding method for measuring the internal pressure of a pipe or container.
The invention will now be described in reference to the appended drawing which shows a preferred embodiment of an apparatus according to the invention.